1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a laser beam printer and more particularly to an image forming apparatus capable of forming multi-color images on both sides of a recording material.
2. Related Background Art
In the image forming apparatus designed for forming a monochromatic (for example black-and-white) image, such as the copying machine, it is already widely conducted to form the images on both sides of the recording material, in view of saving the resources and the space for image output. In a two-side copying machine, an image is at first formed and fixed on one side of the recording material, and then another image is formed and fixed on the other side of the recording material.
On the other hand, with the increase in complexity, diversity, and amount of information in recent years, the use of colored image is spreading and there is emerging a demand for forming full-color images on both sides of the recording material. However, such formation of full-color images on both sides of the recording material will result in perturbation of the image if the conventional method for monochromatic image is applied.
More specifically, in the formation of a full-color image, the desired color is obtained by mixing, by fusion under heating with fixing means, powered toner images of plural colors provided on the recording material. If an image is formed with this method on the first side of the recording material and another image is then formed in a similar manner on the second side, the heat applied at the fixation of the image on the second side is transmitted to the first side, whereby the properly mixed image on the first side is excessively fused and becomes defective.
Also, even if an adequate amount of heat is supplied to the second side from the fixing device, the multi-color image already formed on the first side takes away a part of said heat, so that the color mixing and fixation of the image on the second side cannot be conducted sufficiently, and a satisfactory image cannot therefore by formed also on the second side.